RMH and flue fires?

Posts: 4

posted 5 years ago

Hi there fellow rocket scientists,

After having initial success with testing a combustion chamber and getting a sideways burn in my brick mockup behind the shed, I was ready to get all the materials for the full blown version in our off grid house. I got everything I need except the 10m or so of flues. The guy behind the counter of the business I went to to get my flues ask me what I needed the flues for and after telling him, he told me to get lost. According to him such a system would be prone to soot build up and eventually cause a flue fire and there would be no way for him to let me build it. Even after I told him that the flue would be basically buried in literally tons of rock essentially and that the exhaust gases at the end would be hand warm he still refused to sell me my flues.

Well - I know better next time than to burden the poor man behind the counter with the truth but out of curiosity: is there a potential higher risk of flue fires with RMH?

Location: Southern alps, on the French side of the french /italian border 5000ft high Southern alpine climate.

66

posted 5 years ago

Hi Steve, i don't know first hand. I've made my first rocket last automn. But, looking at how clean the pipes an inside are, i think a clean of the pipes every three or four years might be good, but no more. Few things tho. Bigger systems might burn cleaner than mine, so what i said might not even be valid. And secondly, soot has an ignition point of about 600C°, and exhaust gases of a RMH after the barrel don't reach this temp imho.

Creosote is typically what catches fire (first) in a flue, not soot.. A well built rocket stove won't build up creosote, 'cause it'll burn it as fuel BEFORE it gets the chance to build up.
The counter guy CLEARLY doesn't know what he's talking about.

I am always going to the local hardware stores for something I intend to use for something else. For example. I may buy a brass 1/8 TPI coupling and re-thread it on my tool lathe to use as a a fitting for an oiler on another antique lathe. The high school kid working that dept has no idea...

SO when they come up and ask what I'm looking for I tell them and ask where its at. When they ask what my project is I tell them," The patent has not been approved yet and my patent attorney won't let me talk about it for fear someone will be able to claim prior knowledge and void my patent once the approval process is completed." They usually just show me where the part is and then go away and leave me alone.

... I tell them," The patent has not been approved yet and my patent attorney won't let me talk about it for fear someone will be able to claim prior knowledge and void my patent once the approval process is completed." They usually just show me where the part is and then go away and leave me alone.

I really like this line. Gonna have to remember that one.

To address the original question,
a) - Yes, there is a risk of creosote buildup in any 'cool' flue which is why woodstove chimneys must exhaust at 350F or hotter, and why I get edgy whenever someone wants to do a 'rocket mass heater without the mass.'
b) - No, if you build the RMH to proper proportions, and burn seasoned wood in it (no motor oil or wet punk), the exhaust will not include a significant amount of creosote - only a fine powdery soot. We recommend annual cleaning but some designs can go for 5 years or more without being opened up - fly ash becomes a problem before the soot or creosote does. This is why masonry heaters can exhaust at lower temperatures than woodstoves (90 degrees, which may be 90F or 200F depending on the area of reference), and have interior channels of rough firebrick rather than slick wipe-down surfaces.
c) - All the previous posts are pretty much on point.

On Ernie's and my projects, we protect from this danger in several redundant ways: by (in order of gas flow)
1) Burning the fuel as completely as feasible so there isn't creosote, in a specially-proportioned J-shaped burn channel
2) Providing the air necessary for combustion, at a rate controlled by the wood and the combustion unit's temperature, so there isn't a lot of excess oxygen or fuel going down the pipes one after another,
3) Running the exhaust up a hot chimney stack and down a downdraft area, so the exhaust leaves this area too cool and too full of 'fire-extinguisher gas' to ignite any remaining creosote,
4) Surrounding all the channels with a solid 4" or more of masonry, so even if it caught on fire you couldn't tell,
5) Installing any through-roof or through-wall exits in a fire-safe way (just like for a woodstove, if it's vertical; or through a non-combustible cob wall) so that if someone did light a bunch of creosote on fire inside it just to see what happened, nothing would still happen. (Not everyone does this, as the exhaust temperatures are a lot more like a dryer vent and it's hard to mentally justify the extra $300-$1000 to use the UL-approved woodstove solutions.)

If you change any of these features, you become progressively more vulnerable to the creosote problem.
Examples of things people want to do, that violate these protections (DO NOT do more than one of the following, and seriously consider not doing any of them):
1) AVOID: Swap a big ol' woodstove box for that dinky little weird J-tube burn chamber
2) AVOID: Add extra air feeds, pre-heat the air, enrich the fuel, burn painted or soggy-wet wood, or add fans or other draft boosters to monkey with the mix and flow rates.
3) AVOID: Run the exhaust past something cold, like a water jacket, or out the top of the barrel, instead of waiting until it's done its time in the combustion unit
4) AVOID: Leave the ducts exposed to air, or surrounded in porous materials like sand or rap-rock (if this is done, the material should be stovepipe, not ducting.)
5) AVOID: Use plastic or air-handler ducting for through-roof fittings or through-wall exits. In principle, if you've done all the above properly, the exhaust should never catch on fire. In practice, people often want to light a wad of newspaper or a log of 'flue-cleaner' in the chimney on ceremonial occasions, or put in a new woodstove when they take the old weird one out, and something that looks like a chimney but isn't is an accident waiting to happen.

Creosote fires in ill-designed metal flues are deadly, nasty, traumatic things. The creosote coats the flue unevenly, and then when it lights off, the flue expands unevenly, often warping free of its joints and spraying very high-temperature flames and black smoke into the room. Ducting will actually melt at this point, whereas stovepipe will generally just get red hot and glare.

That said, I don't believe your dude had ever heard of masonry heaters.
Given that they are typically built out of masonry, not ducting, he might still have felt he had a point.

The kind of wordplay required to buy parts from someone without alarming them reminds me of the original building methods, which are even less acceptable under building codes: recycled materials.

It is not legal to re-sell used stovepipe, except as scrap metal (you can't sell it to someone for use with their woodstove, or any use where air or smoke will flow through it). This is mostly to protect people from leaky, damaged, gunked-up, pre-creosote-coated, or pre-toxic-gick-mystery-coated pieces of junk being foisted off on the largely ignorant public. Nobody can handle the liability of telling someone else that a chunk of stovepipe is 'safe' to use. There is still a black market and grey market for 'never-used surplus' or 'lightly used,' especially the triple-wall metalbestos sections that are like $100/foot new.

If you want to buy something and use it outside its socially and legally accepted lifespan,
Your choices are
1) to lie, either directly or by implication;
2) to find some form of the truth that's acceptable to you and your seller ("It's for forms for a giant clay project, do you have any 8" diameter cardboard or metal tubes" or "I'm changing around my central heating - yes it's a wood-fired heater, but this is for the heat-exchanger not the fire part." "It's a masonry heater not a woodstove; woodstoves are under 900 kilograms and this is in an exempt category; it works differently.)
3) to say as little as possible about the project, and be specific only about the parts needed
4) to shell out and buy the more expensive parts if available (it only doubles the cost for most of the system)
5) to keep shopping around until you find a supplier who cares more about how things work than about liability, and is able and willing to listen and learn new things. These guys generally have more useful recommendations on stuff, too. Junkyards are more often flexible than retailers and even recycling depots.

There are some young idiots out there, but there are also a lot of well-intentioned, 'experienced' engineers and builders who have spent 30 years in the industry and barely even heard of a Rumford fireplace, let alone a contraflow masonry heater. They have literally been working inside the box, and the box is a woodstove with a UL-approved manufactured chimney, or a box-shaped masonry fireplace with approved rebar-counts in the approved foundation. And there are, I hate to say it, a few little bosses of little empires who are very comfortable with the power to approve or deny other people's ideas, without ever trying them.

If you try to build a radically different item while staying 'inside the box,' you will end up spending a couple of decimal points more to please everyone. Our first attempt at a code-compatible RMH cost about $1800, when previous attempts had cost more like $200-300 including delivery of sand. Almost $1000 of that was for the double-wall to triple-wall chimney, through-roof, etc; and we still re-used some existing parts rather than buy all new. Another $150-300 for permits, $300 for sand and perlite, and a few hundred for recycled and decorative bits. I suspect the proper permit, which we retroactively were told we didn't get, would bring the price up maybe double that again - it has to go past the alternative appeals board, not just the lady at the counter with the rubber stamp.
If you replaced the cob with concrete and 'conventional' masonry, and all the metal with stovepipe, and the barrel with a certified-something-or-other, you'd be back in the ballpark of $6,000-$10,000 like a DIY masonry heater kit. Up to as much as you care to spend, if you go with soapstone and all that.

If you want to build a $50 heater using recycled and earth-sourced materials, you are not going to be buying from people who give a hoot for the building codes.

You can actually make the whole thing from earth, except the barrel. You can use bags or boxes for forms, and shape the cob into channels, it just takes longer and is touchy to clean. Or you can make straight-walled channels and bridge them with bricks. Takes more skills and bricks. Ducting is just the easiest way to assemble the 'voids' in a continuous flow path and flow area size. And it's cheaper than stovepipe.

Erica nice summation. You missed a couple of points though. 1. given the fact that the RMH has such a fast batch burn time you are more likely to be present if a chimney fire starts and you should be able to snuff most of it but simply covering the air intake. Which brings us to 2. Chimney fires are dangerous because they turn your chimney into a giant rocket stove with a 10 to 20+ ft chimney. If your run is mostly horizontal it doesn't have that height to draw on if it gets going.

C. Leitellier : Take a look at the Thread extension post by Satamax Antone above (May 20th,2012 ) Creosote, has a Ignition temperature of 335dC, or
~570dF~, this is well below the lowest temperatures the rocket mass heater will be working at within 5 -10 minutes of starting operation, indeed in the
manner that the pyrolyzed wood gases get consumed, the best analogy to explain the High heat energy, high efficiencies of the RMHs burn is to say that
we are perpetually running an ongoing 'chimney fire' within the Burn Tunnel and Inner Heat Riser !

Any attempt to 'snuff out' the ongoing fire in a rocket mass heater after the first 5 - 10 minutes by capping off the top of Feed Tube will create an Oxygen
deficient Environment in which continued production of Pyrolyzed wood gases with greatly increased levels of Carbon Monoxide, (by itself a very flammable
gas), and this will continue to happen at the extremely high temperatures of the Combustion Zone.

Because the Flammable gases are initially above their ignition temperatures, Once you have capped the top of your Feed Tube, you will not be able to use your
rocket mass heater RMH for several hours until it has cooled down below the ignition temperature of ALL of the flammable gases ! (and remember the bricks of
the combustion chamber are surrounded by insulation!)

Premature removal of your cap, will result in the mixing of 'fresh oxygen' with the Hot Pyrolyzed Wood Gases and Carbon Monoxide. The resulting extremely
rapid combustion of these mixed gases is something the Fire Services call 'Flash Over', this would occur at temperatures above the ignition temperature of your
Clothing and Hair !

Actually I do not have a suggestion for dealing with a Fully operational fire in a rocket mass heater besides careful monitoring, while you could use fire place
tongs to pull the fire wood out of the Burn Tunnel, placing it into a metal pail and then carry the smoking embers out of the house, simply never getting your
self in a situation where that would be necessary works for me!

I would also never recommend pouring water inside a RMH ether ! For the Craft Big AL !

Hello Erica. first I would like to say thanks to you and Ernie for helping to pioneer this field and bring people everywhere a very inexpensive and simple means of using twigs and windfall to keep themselves warm.

I have a couple questions regarding this topic.

Erica Wisner wrote:

a) - and why I get edgy whenever someone wants to do a 'rocket mass heater without the mass.'

I don't understand the problem here. Are you saying that without the mass, then the exhaust gasses will be hotter, and more likely to start a flue fire?

What if one has an identical system with no thermal bench, but has run horizontal pipes which release most of the heat into the space on demand, but the only difference is the heat is not being stored in a thermal mass. does this matter? Or what if in the same exhaust run where the cob bench would be, someone has cooled the exhaust with a water coil to the same temperature it would have been with a thermal bench, will this make a difference?

To clarify, if the riser is the same heat, and the barrel is the same heat, and exhaust leaves the house at the same temperature, does it sill matter if it goes through a huge mound of clay or not?

Or are you saying that the huge mound of clay would easily mitigate a creosote fire inside, as opposed to a piece of stove pipe which would obviously fare worse?

In which case I would understand the logic there.

Also to clarify a seperate issue, is it not okay to run exhaust straight out the house gas heater style? I saw a video where people had done that either in one of your workshops or one where paul wheaton was involved. And they were marveling at the low heat exiting the side of the building and just steam and c02.

Is this bad? Is the urpose of running the chimeny vertical after it leaves the exterior wall and raising it up above the roof in case there is a flue fire, so that is the safest place to have a fire exit the system? rather than down under the eaves?

I thought the different lengths of chimneys I had seen was just to get more draft if it was needed, but if there is also a safety reason for running the chimney up past the roof line, I will do it.

I guess my understanding was that if the burn chamber and riser and barrel were functioning properly, creosote is not an issue so the only difference between having mass downstream or not would be how much heat do you want to recapture before it leaves the building, and also that the mass creates a more stable temperature.

Erica Wisner wrote:

1) AVOID: Swap a big ol' woodstove box for that dinky little weird J-tube burn chamber
2) AVOID: Add extra air feeds, pre-heat the air, enrich the fuel, burn painted or soggy-wet wood, or add fans or other draft boosters to monkey with the mix and

I can understand why one would not want to do this willy nilly, but what do you think of Peter Berg's batch box?

Do you think it is possible if done scientifically, there is a method to add a larger fuel capacity while still meeting the requirements of an efficient burn using a "rocket" type riser, and external barrel?

Because I was hoping to copy his design. Do you think his batch box design is a creosote hazard that the conventional RMHs do not pose?

Since peter berg's batch box gives people the option of not having to babysit their feed tube constantly, it seems like a worthwhile pursuit.

Lastly, if one does fore go the mass and instead just has stove pipe exposed to open air inside the dwelling, would simply checking for creosote often solve the whole flue fire conundrum?

I really thought that the complete burning of the wood eliminated the creosote issue.

I'm in the Class A chimney pipe camp. I know it costs a lot more than venting a gas heater, but you are risking your house if anything goes wrong. The Class A starts anywhere that wood is within 18" or at the ceiling of the room where the wood-burning device is.

Matt Walker has done a lot of work with Peter's batch box; Satamax Antone is also a big fan. Matt entered the batch box in the Woodstove Decathalon last Nov. in DC. It was not the most efficient unit there.

www.dragonheaters.com
http://blog.dragonheaters.com/

jacob green

Posts: 29

posted 4 years ago

Cindy Mathieu wrote:Jacob,

I'm in the Class A chimney pipe camp. I know it costs a lot more than venting a gas heater, but you are risking your house if anything goes wrong.

Of course. Safety is a prerequisite. Cost is a luxury. Especially if the entire stove can be built really inexpensively, and especially if by eliminating the mass, there is more risk of a flue fire, then it makes perfect sense to use an exhaust system that is as close to foolproof as one can get. It really makes perfect sense regardless, to use high quality components.

I still am unclear on how the "massless" aspect of alternative RMH designs is more likely to create a flue fire, assuming that the burn chamber, the riser, and temperature leaving the structure is the same as an identical RMH that features a giant thermal bench.

I understand how a system with random dimensions and random inventions thrown in on a whim could create a flue fire. I understand that any variables which result in incomplete combustion could build up creosote and are more likely to create a flue fire.

Such as insufficient riser temperature, wet wood, incorrect fuel to air mix etc.

Cindy Mathieu wrote: Matt Walker has done a lot of work with Peter's batch box; Satamax Antone is also a big fan. Matt entered the batch box in the Woodstove Decathalon last Nov. in DC. It was not the most efficient unit there.

I just looked at that and it brought me to walker stoves. Very impressive.

So that is the same Mathew who was so helpful, and listed out a series of concise answers to my questions? Thanks Mathew. : ) your stove looks excellent!

Actually he was the one who helped me with the important puzzle piece of how the insulated aspect of the stoves are so important, and it finally clicked for me how the process worked.

Prior to that I thought one wanted dense fire brick in the airpath because the brick would heat up so much, it would ignite the unburned gasses higher in the riser

xD

So you say the batch box design was not the "most" efficient. Was it in the running? Sometimes a little efficiency can be sacrificed to acquire a different KIND of efficiency.

One thing that a lot of people tend to overlook when thinking green, is that TIME is ALSO a valuable resource. The more free time people have, the more gardens can be tended to, or bottles recycled, or soup kitchens volunteered at, etc.

I am glad people like Peter Berg and Mathew Walker are trying to go beyond the confines of the box, literally and figuratively, to strive for new heights. Innovation does not happen, from not trying. That I know very well.

Innovation is something I am an expert at, even though I know very little about rocket stoves/heaters etc.

The cold hard reality is, a LOT of people in the mainstream will simply not be interested in wood as a fuel source, if the process requires constant effort. I think it is GOOD to try and come up with new ways of achieving this. And if it fails, try again. And if it fails, try again. And if it fails try again.

a) - and why I get edgy whenever someone wants to do a 'rocket mass heater without the mass.'

I don't understand the problem here. Are you saying that without the mass, then the exhaust gasses will be hotter, and more likely to start a flue fire?

I get edgy too. Remember that many of the iron boxes can get very good efficiency ratings in the lab too. They do so by running them full out, just like a high mass heater, but... they put out so much heat that the room becomes unlivable very quickly... so the highly efficient iron stove in the lab becomes another 10%er in the home where it is run at an idle/smolder. The rocket heater without mass is much the same. you take away the mass and you have an iron box that can not (except for testing) be run at full efficiency without over heating the room. So the user will try different things... ask friends and cut the air flow till it smolders along nicely. Six months later when they have a chimney fire and no insurance, they blame the RMH... except it wasn't really a RMH, it was really another iron box stove.

So just remember not to call it a rocket anything so those who are working on the RMH don't get blamed for problems with the iron box stove that gets built.

I don't know if you got that. It is not that design is flawed and won't work without mass, just that it is not appropriate for home heating and the long slow burns a low/no mass heater needs. The best low mass heater (also smallest foot print) is probably the wood pellet stove. The wood pellet stove requires power and pellets, but if you have those, it is fine. It can control the amount of fuel instead of air and so can burn efficiently even at low settings.

jacob green

Posts: 29

posted 4 years ago

Len Ovens wrote: so the highly efficient iron stove in the lab becomes another 10%er in the home where it is run at an idle/smolder.

Len Ovens wrote: and cut the air flow till it smolders along nicely. Six months later when they have a chimney fire

My question specifically eliminated that eventuality. To make sure I am clear, having a "smoldering" fire is not my intention. I indicated that I completely understand why incomplete combustion would result in creosote, and in turn, flue fires.

"To clarify, if the riser is the same heat, and the barrel is the same heat, and exhaust leaves the house at the same temperature"

What I meant by that was the the fire would NOT be smoldering. The main thing which attracted me to rocket heaters in the first place was the complete burn, and videos of people showing how only steam and C02 was exiting the system. So for me, if I am not getting complete or near complete combustion, I am not interested.

Len Ovens wrote: you take away the mass and you have an iron box that can not (except for testing) be run at full efficiency without over heating the room.

If a smaller volume of fuel was on fire, then the fuel could be burned at full efficiency, and the room would not overheat,

Len Ovens wrote: So just remember not to call it a rocket anything so those who are working on the RMH don't get blamed for problems with the iron box stove that gets built.

Do I have your permission to at least call it a "Pet Rock Stove", or is that too close to "Rocket"? How about "Racquel Squelch"? ( cold burn, not-rocket sound ) : )

Len Ovens wrote:It is not that design is flawed and won't work without mass, just that it is not appropriate for home heating

My interest in non-mass rocket stoves is not for heating a standard home. In a standard home situation I would MUCH rather have a large thermal mass radiating all the time.

In small spaces however, mass, starts to become a problem. Either because of space, or because of weight.

My nature is to try and figure out new solutions, and create new things. Sometimes I succeed. Sometimes there is no room for improvement. Occasionally dogma bares it's fangs.

Have you seen Kimberly Stoves? I saw a video of a guy who had one in his motorhome. Looks awesome. The webpage says it burns for EIGHT hours on one load. Cool, where do I sign up? I would just get one or two of those, except...they are almost 4,000 dollars! Yikes.

The Kimberly stoves I have seen have no mass. And to complicate matters, it is also an Iron Box, well more like a Steel Cylinder. Both descriptions would be equally suitable as 80's metal bands, or porn titles, capitalization was an after thought.

Either way, the stove has no mass, and does not appear to be prone to flue fires. And I am not naive enough to believe that advertisements are real life, but the people enjoying the stoves don't look "edgy" at all, in fact, they look quite comfy. : )

So I was just trying to figure out specifically what aspect of massless designs would cause flue fires. I was not sure if the point was that the exhaust came out a lot hotter, or that the mass itself could tolerate an internal creosote fire, or both.

I have several designs for wood and pellet burning stoves I am working on, a couple don't even resemble RMH in the least. And I am DEFINITELY planning on building one or more RMH stoves in the mean time which follow the known and trusted methods as close to the letter as I can. The current design is brilliant, and brilliantly simple, and I definitely want to know how to build the current design as a LIFE SKILL, and as something I can always fall back on, which I know from other people's experience, will definitely function and function safely. And I am gathering as much information as possible to help with all of these processes.

Constructive input is much appreciated!

: )

Len Ovens

pollinator

Posts: 1457

Location: Vancouver Island

29

posted 4 years ago

jacob green wrote: "To clarify, if the riser is the same heat, and the barrel is the same heat, and exhaust leaves the house at the same temperature"

What I meant by that was the the fire would NOT be smoldering. The main thing which attracted me to rocket heaters in the first place was the complete burn, and videos of people showing how only steam and C02 was exiting the system. So for me, if I am not getting complete or near complete combustion, I am not interested.

The exhaust will not leave the house at the same temperature for mass and no mass, all other things being equal. It will be much hotter and that heat/energy would be lost. However, if the flue is used that way all the time, there is no special reason it should start a fire in the flue. It sounds like you intend to be the only operator of the heater. That would make it much safer.

If a smaller volume of fuel was on fire, then the fuel could be burned at full efficiency, and the room would not overheat,

Yes. J-tube rocket burners are hard to adjust the fuel amount in, they are designed to work best at one burn rate. An L-tube rocket stove (as in for cooking) is easier to adjust, but requires more frequent attention. An L-tube also can work ok at smaller burn chamber/riser sizes... especially with short flue runs as would fit a no/low mass heater. I have seen as low as 3inch work.

Do I have your permission to at least call it a "Pet Rock Stove", or is that too close to "Rocket"? How about "Racquel Squelch"? ( cold burn, not-rocket sound ) : )

I deserved that. I think it would still be a rocket stove. Have you looked at the pocket rockets? They burn clean but send a lot of heat outside. Look anyway, but I think a smaller stove would work better.

In small spaces however, mass, starts to become a problem. Either because of space, or because of weight.

Yes, I can see that. Do you have a specific application in mind? What size/weight are you designing for? I have thought of small scale rocket heaters, but have not had the materials/money/time to try my idea out. Personally I would try to include some mass in the form of soapstone or cast iron if nothing else. These are denser materials than cob or even fire brick and so would store more in less space. Because you are running small, I would try to run hotter (well low mass kinda makes it that way any way) and have a thin insulating layer on the outside... at least for after the heater stops burning, but maybe even while burning and run many small burns rather than continuous. The other thing I really want to try, is to use a phase change material. Sulfur though in the right melting temperature range tends to eat steel but Tin would be nice. So a steel box full of Tin could be lighter than steel for the same heat storage provided you got it above melting temperature. There are some solar cooker projects that have used a box of Tin in an insulating box to heat food after the sun has gone down successfully. While there are other phase change materials around, most are designed for much lower temperatures. Tin works well at wood stove temperatures.

(Re)movable mass is another interesting idea. In days of old chunks of soapstone were used this way. Put into the fire box and then used for cooking in a strawbox after the fire was out or could be used as a radiant heater in another room. A large pot of water would work the same way though the temperature would be limited to 100C so even a thin insulating layer would probably not make sense. The box of Tin should work really well and an insulating layer would help it last longer while heating a small room nicely.

More than one movable mass could be used one after the other till you reached the weight limit. It might extend the weight limit by allowing the weight to be spread out. Though if the home moves often the extra weight would cost more fuel to move, but if the floor weight bearing is the limit, it could work. It is a manual system that requires human intervention reasonably often. I can think of self feeding ideas for the fuel, but moving mass around is all manual and knowing when to move it means being in the home while the heater runs.

Have you seen Kimberly Stoves? I saw a video of a guy who had one in his motorhome. Looks awesome. The webpage says it burns for EIGHT hours on one load. Cool, where do I sign up? I would just get one or two of those, except...they are almost 4,000 dollars! Yikes.

I was going to suggest using a gasifier set up next... it turns out that is what the Kimberly is It is hard to tell as their web page doesn't tell you much (so you don't make your own) but it appears the wood is smoldered at as slow a rate as possible and the fumes from this are reburned. The reburn is where most of the heat comes from. Think of a tall firebox with the wood at the bottom. There is just enough air to let it burn a slow smudgy fire. They try to keep the temperature just above wood flash point (they say about 450F). Then above that they have another air intake that is used to burn the smudge at somewhere over 1000F (probably not much over as they use b-vent) which is the heat that is usable. It is more like the rocket than they would like to admit They claim they paid $40,000 for certification in such a way that makes it sound like that was the major development cost or maybe they didn't keep very good track of dev costs or it was a spare time project. There are some small gasifier cookers around the net and I would guess this was based on that. The big "patent" word is missing so it would seem that is not even applied for which would indicate it is based on known technology... feel free to build your own and sell it or post plans or whatever. I would guess also that they use some interesting channels after the reburn to make sure as much of the heat hits the room as possible... still not enough to use single wall vent like gas heaters do... though that may be more to satisfy code than need. Not a lot of bad things to say about it. My main thing is that because it does not use mass at all the surface temperatures are going to be higher:
- more heat will go through the walls no matter how good the insulation is (do the math... there is a thread about that already).
- dust in the air gets burned and can cause breathing/health problems.
- there are more opportunities for skin burns.
Maybe not with the standard RMH with the proud barrel. Mine has mass right up the barrel

Anyway, the owners manual is a free (with no sign-in) download.

So I was just trying to figure out specifically what aspect of massless designs would cause flue fires. I was not sure if the point was that the exhaust came out a lot hotter, or that the mass itself could tolerate an internal creosote fire, or both.

It is as I said, the possibility for misuse is the problem. not the design itself. In a one person tiny home that may not be a problem... in any house where there may be people operating the heater who have prior wood stove experience there is a danger. Even masonry heaters have a very real operator problem even with the builder getting a signed paper saying the buyer has read and understands the operating instructions.

I think for small area use, the gasifier may be the right thing. The fuel is restricted with gasifier air intake allowing normal wood to be used instead of pellets. There is a good layer of insulation in the back of the unit to allow installing closer to walls, but the front radiates as much to the room as it can so the exhaust can be cooler.

jacob green

Posts: 29

posted 4 years ago

Len Ovens wrote:

Do you have a specific application in mind? What size/weight are you designing for? I have thought of small scale rocket heaters, but have not had the materials/money/time to try my idea out. Personally I would try to include some mass in the form of soapstone or cast iron if nothing else. These are denser materials than cob or even fire brick and so would store more in less space.

A couple applications. One would be any type of livable vehicle from small to large motorhome.

Another would be anywhere one might set up temporary camp, such as fishing expedition, gold panning, etc. Small lean to situations. Occasionally there is a need to spend some time in a small one room cabin. etc.

One of the ideas I had was to fill an 8 foot, two inch diameter tube, laying horizontally, or slightly diagonal, with pellets/twigs and figure out how to make the fuel burn very slowly along the entire length of the tube, so that about only 1 inch of the metal would be getting hot at one time. The entire tube would either sit horizontally, or diagonally, and function as the feed tube, burn tunnel, and riser ALL IN ONE straight pipe. Or it could have an additional 3 foot riser on one end to create draw. One could bury this in dirt under a sleeping bag or cover with dirt on the side in a lean to. But now I understand rocket heaters better, so I know there would be no way to get hot enough burn to be efficient this way.

But that's just to give you an idea of how radical some of the directions I am exploring are.

Possibly if I could figure out how to burn the resulting woodgas at the end of the system with a small chamber it might work. Either way, I would not want it to accumulate wood gas and create a pipe bomb right next to me.

maybe I could start a fire at both ends of the 8 foot tube, and the at least the woodgas from one end would have to travel through the fire at the other end to exit the system.

Len Ovens wrote: (Re)movable mass is another interesting idea.

on the 8 foot tube stove I had an idea to have semi-circular cement drain tile underneath the tube, then cover the tube on top with several pieces of the same material to complete a cement jacket around the entire burn tube. Because it would quickly get too hot, then the cement jacket would simultaneously block the extra heat, while storing it for later.

Or when the space got warm enough after initially starting the fire, I could cover with dirt and rocks.

Regarding the normal 4.5 to 6 inch RHM with small mass such as over an existing fireplace hearth. For portability I wanted to try 5 gallon cans filled with water, stacked very close to the barrel and vertical exhaust. I figured for a small space they would block some of the initial heat that would be too hot for small space, but would absorb plenty. I could get 10 5 gallon cans stacked two high around a 20 gallon barrel and vertical exhaust leaving the front of the barrel exposed to direct heat. The cans could simply be emptied of water, and with lids off they would stack and be very light and take up very little space. I would either leave holes in top to vent if they got to hot ( which I doubt they would, or create a system of tubing to vent outside if it was creating a steam problem in the space.

Len Ovens wrote: I was going to suggest using a gasifier set up next... it turns out that is what the Kimberly is

I am starting to think this is the only way to achieve a very small volume fire, and at very slow speeds, and have complete burn.

That is another one of my objectives to meter the fuel out VERY SLOWLY. It does not take much heat at one time to take the edge off in a very small space. And metering out heat very slowly is contradictory to a complete, efficient burn rocket style. But since gasifying works by INTENTIONALLY making a very slow, cold burn, one can achieve a much smaller btu per hour, while simultaneously extending burn time.

Which for small spaces is perfect. Because if one does not have the MASS to accumulate the energy from a very fast, hot burn, the only other option is to slow down the burn to achieve a the goal of not having to constantly put fuel in.

Unfortunately the gasifier setup seems a lot more scientifically complex. Where as I got my mind around the physics of a rocket heater very quickly, I am not certain what is required to build a kimberly type of gasifier.

The guy in the video with the motorhome did mention that at the top of the stove was a "honey comb" thingy, which I can only assume is a catalytic converter, which would make sense, seeing as how the fire is being metered out so slowly and with such a cold burn, that the exhaust is mostly wood gas.

I have to admit, that playing around with woodgas in chambers, seems a lot more technical, and critical, than achieving a quick clean burn based on the simple physics in a conventional RMH.

But there cant be that much to how the kimberly stoves work, and at $4,000 there will probably be a lot of motivated people trying to figure out how to build a reasonable facsimile.

Len Ovens

pollinator

Posts: 1457

Location: Vancouver Island

29

posted 4 years ago

jacob green wrote:

One of the ideas I had was to fill an 8 foot, two inch diameter tube, laying horizontally, or slightly diagonal, with pellets/twigs and figure out how to make the fuel burn very slowly along the entire length of the tube, so that about only 1 inch of the metal would be getting hot at one time. The entire tube would either sit horizontally, or diagonally, and function as the feed tube, burn tunnel, and riser ALL IN ONE straight pipe. Or it could have an additional 3 foot riser on one end to create draw. One could bury this in dirt under a sleeping bag or cover with dirt on the side in a lean to. But now I understand rocket heaters better, so I know there would be no way to get hot enough burn to be efficient this way.

Have you looked at sawdust burners? It doesn't sound like it, but they actually burn quite clean. Packed sawdust around and empty centre the sawdust acts as its own insulator and the saw dust burns from the center out. I have seen small ones for cooking and barrel/drum ones for heating.

Len Ovens wrote: I was going to suggest using a gasifier set up next... it turns out that is what the Kimberly is

I am starting to think this is the only way to achieve a very small volume fire, and at very slow speeds, and have complete burn.

Unfortunately the gasifier setup seems a lot more scientifically complex. Where as I got my mind around the physics of a rocket heater very quickly, I am not certain what is required to build a kimberly type of gasifier.

There seems to be a tin can build that works well for cooking. In fact I think there is a video in one of these threads.

The guy in the video with the motorhome did mention that at the top of the stove was a "honey comb" thingy, which I can only assume is a catalytic converter, which would make sense, seeing as how the fire is being metered out so slowly and with such a cold burn, that the exhaust is mostly wood gas.

That would be a catalytic converter to scrub any left over unburnt fuel from the flue gas. I think the gasifier is simpler than it seems. There is a very small burn chamber on the bottom. All the wood goes in there. Over that there is a plate with holes in it. This acts to keep fresh air from going down into the primary burner. As the wood gas rises above that plate heated air is introduced and the wood gas ignites, this is the flame you see through the window. I don't know if this reburn part is insulated or not, they seem to want to keep all of it a ways from walls and the warn that the outside surfaces are too hot to touch. It is hard to know what the 80% efficiency means as wood burning appliances have traditionally (and legally) been measured against 84% or so because the last 16% is needed to get the flue gas safely out the top of the chimney. So the 80% is either very good or so-so.

I'm tired... going to sleep.

Len Ovens

pollinator

Posts: 1457

Location: Vancouver Island

29

posted 4 years ago

jacob green wrote:

Unfortunately the gasifier setup seems a lot more scientifically complex. Where as I got my mind around the physics of a rocket heater very quickly, I am not certain what is required to build a kimberly type of gasifier.

For small and long clean burn try a Sawdust Stove. One 5 gallon bucket with one hole cut in it, two plastic tubes used only for loading, and some sand. One bucket full of sawdust lasts 8 hours and burns clean. This is a Rocket stove that uses the fuel to form the feed and riser and slowly burns itself out. The finer the sawdust (and tighter the packing?), the longer the burn. I have seen these smaller and larger. I think this predates the rocket stove/heaters.

The gasifiers I have seen on youtube are similar to This one. I think the one I showed you is about the best, but it suffers from the same problem as all of them. I don't think any of them would last as long as the Kimberly with the same fuel. These are all uncontrolled. They are effectively the same as the rocket stove/heater in having a primary and secondary burn. They are batch devices rather than the Rocket continuous feed (manually), but an hour burn is still not long. I think the principle is the same in the Kimberly though, just that the primary burn is much more controlled and out of site. I think the primary burn in the Kimberly is quite small (less than a cubic foot?). Under the windowed area and the windowed area is the secondary burn area. The secondary burn area probably has a carefully determined air intake as well... While too much air will not hurt the burn, the extra ballast gas will take more heat up the flue. So the complex parts really are the two air metering holes. The primary burner may be designed also to make it hard for all the fuel to burn at once.

jacob green wrote:Hello Erica. first I would like to say thanks to you and Ernie for helping to pioneer this field and bring people everywhere a very inexpensive and simple means of using twigs and windfall to keep themselves warm.

I have a couple questions regarding this topic.

Erica Wisner wrote:

a) - and why I get edgy whenever someone wants to do a 'rocket mass heater without the mass.'

I don't understand the problem here. Are you saying that without the mass, then the exhaust gasses will be hotter, and more likely to start a flue fire?

What if one has an identical system with no thermal bench, but has run horizontal pipes which release most of the heat into the space on demand, but the only difference is the heat is not being stored in a thermal mass. does this matter? Or what if in the same exhaust run where the cob bench would be, someone has cooled the exhaust with a water coil to the same temperature it would have been with a thermal bench, will this make a difference?

To clarify, if the riser is the same heat, and the barrel is the same heat, and exhaust leaves the house at the same temperature, does it sill matter if it goes through a huge mound of clay or not?

The huge mound of clay/masonry does two things: stores heat, supports and double-seals all those cattywompus pipes. (There's a family word I've never tried to spell before!)
Without that double-seal I would be concerned about air/smoke leaks over time, and I would also wonder about whether the pipes would be more prone to over-heating (and to under-heating, meaning they could condense more moisture and creosote than an up-to-temperature bench). I would definitely not use flimsy ducting in place of stovepipe, without the support of the monolithic masonry. The ducting's great as formwork in a context where the mass keeps it below certain temps, but exposed to air and potentially higher heats you could off-gas the galvanization and some types of "high-temp" foil tapes as well.

Or are you saying that the huge mound of clay would easily mitigate a creosote fire inside, as opposed to a piece of stove pipe which would obviously fare worse?

That too.

In which case I would understand the logic there.

Also to clarify a seperate issue, is it not okay to run exhaust straight out the house gas heater style? I saw a video where people had done that either in one of your workshops or one where paul wheaton was involved. And they were marveling at the low heat exiting the side of the building and just steam and c02.

Is this bad? Is the urpose of running the chimeny vertical after it leaves the exterior wall and raising it up above the roof in case there is a flue fire, so that is the safest place to have a fire exit the system? rather than down under the eaves?

I thought the different lengths of chimneys I had seen was just to get more draft if it was needed, but if there is also a safety reason for running the chimney up past the roof line, I will do it.

In some (unusual, short) buildings, you can get away with a substandard chimney.
(Incidentally, this is also true for woodstoves, and I've seen some poor performers hooked up to ridiculously under-built "chimneys" because, for example, installing a proper-height chimney in a vinyl yurt was beyond the occupants' budget or interest. In that context the best place for the chimney is occupied by a "star-gazing" skylight, and any other location involves building an independent, self-supporting, insulated "chase," basically you live in a tent and the chimney gets its own, tall house.)
Sideways chimney exits were favored in the original Evans designs because their cob cottages had non-combustible walls and unconventional living roofs lined with combustible EPDM pond liner. And for cost; probably primarily for cost. It's worth noting that the little 5' or 6' snorkel-style chimneys on Ianto's buildings may still be the highest effective opening in that house, because the roof is literally air-tight and the cob walls give a nearly-airtight seal to those low eaves. Just getting up above the eave on the downwind side can be sufficient in those buildings (they have a mild, sheltered valley climate, too, with predictable valley wind directions).
If working with a low-height building with most of these advantages, clever and resourceful fire-apes can probably figure out a substandard chimney that will still work OK, thanks in part to the heat riser draft's push. If it's a "push" system you also need to be very careful about detailing to prevent all possible smoke or CO leaks into the house, the exhaust is visibly cleaner than woodstoves but it's not breathable air.

In many (conventional, taller, roof-vented) buildings, the building itself will generate wind eddies, pressure differentials (hot stack effect when it's warmer than outside air), and other adverse forces that can cause an efficient solid-state heater to backdraft.
Many climates also have unpredictable wind directions that make the idea of a "downwind" draft location impossible. Conventional chimneys create their own "downwind" effect using the (venturi or bernoulli? things moving across an opening tend to create a lower pressure that sucks fluid out of the opening, I think it's venturi), which means they work best when pointed straight up, above any local disturbances, so the wind from whatever direction will most likely flow sideways across the chimney opening. In climates or building sites with variable or gusty wind, any exhaust below the roof elevation will sometimes catch gusts and pressure-waves and eddies off the structures, creating increased pressure and backdraft conditions. Even if it's capped, the pressure may build up on the windward side, or at the wrong point in a pressure-wave swirling around a corner, and natural draft only goes from higher pressure to lower, it can't push uphill.

There may be no conceivable way to successfully mitigate all these forces in some buildings short of a mechanical fan (hard to calibrate to the varying burn rates throughout a fire for adequately clean burn), or a conventional chimney with enough exhaust heat to draft warmly (above the dew point of the water in the exhaust). I recommend making allowances for the idea that you may want a conventional chimney exhaust. For example, by building a T cleanout-port near the end of the bench where you might later retrofit a proper chimney.

If you have an existing chimney of appropriate size, by all means I would use it, and not bother with through-wall detailing.

The problem with substandard chimneys as the default option is the fact that it's more difficult and expensive to convert a poor through-wall exhaust into a proper chimney than to install a proper through-roof chimney, in most cases. All exposed chimney outside the house will need insulation (and more wasted exhaust heat) to draw upward, and even with the best insulation it will lose heat to outdoors and won't draw as well as it could have as a cheaper, single-walled vertical exposed to the indoor warmth. indoor chimneys share heat with the home (which we want); exposed outdoor chimneys lose heat, and require more excess exhaust heat to draft properly.

Also, detailing a through-wall so that it doesn't channel water runoff into the wall cavity turns out to be even more difficult than detailing a through-roof properly so it doesn't leak. And there are usually better kits available for the through-roof!

So if you want to go with a sideways exit, I'd suggest doing it relatively high on a gable-end wall (better draft and no ice-dams or eave runoff to deal with), and be prepared to convert to a full-height chimney if needed after initial testing.

I guess my understanding was that if the burn chamber and riser and barrel were functioning properly, creosote is not an issue so the only difference between having mass downstream or not would be how much heat do you want to recapture before it leaves the building, and also that the mass creates a more stable temperature.

Hopefully creosote would not be an issue, but smoke leaks and the performance still would be. As commented by the others before I got back to you.

Erica Wisner wrote:

1) AVOID: Swap a big ol' woodstove box for that dinky little weird J-tube burn chamber
2) AVOID: Add extra air feeds, pre-heat the air, enrich the fuel, burn painted or soggy-wet wood, or add fans or other draft boosters to monkey with the mix and

I can understand why one would not want to do this willy nilly, but what do you think of Peter Berg's batch box?

The smaller batch boxes seem like a temptation to treat the whole thing like a woodstove; I've seen novices over-feed Matt Walkers' batch box and jam up the port, resulting in ongoing clouds of smoke until the whole thing burned down. In an indoor mass heater, or especially the metal-only radiator idea, clouds of smoke translates to creosote lining the pipes or bell surfaces that will be hard to remove.

Again, we design the stove not to put out smoke and creosote, and then we design it so that it can survive a creosote fire ANYWAY, because we know pumping exhaust through low-temperature heat exchangers is a recipe for distilling whatever little dregs of creosote may escape. This is the legacy of half-a-million years of fire-ape evolution as the survivors of repeated, catastrophic, home- and city-destroying fires. When we respect, honor, and court the best understanding and practice of fire-tending, we get amazing results that satisfy deep-seated needs, cravings, instincts, and mental fascinations. When we get too complacent about living with the red beast, it leaps up to threaten us and everyone we love.

Aaand after that etiological digression,
I also got to see Peter's 8" batch box thingy at this fall's Innovators' Gathering. It was awesome. The cleanest burn I've ever seen, or heard of, cleaner than a candle. Granted it was lit and operated by Peter himself, and not by a bunch of enthusiastic helpers like Matt's poor thing, but it's a great example of what an intelligent human being can do with conscientious care and sophisticated analysis tools, to produce something that succeeds in scaling up.

The batch-box at 8" is a big beastie, that is better for a large shop than a small shelter. It unfortunately doesn't have quite the same amazing performance at smaller scales, the exhaust emissions tend to spike from time to time due to uncontrollable (or unfathomable) variations in the fire's progress.

I would definitely say that the batch boxes Peter is working on count as rocket mass heaters, burning clean enough to be reasonable engines for mass heat storage. Look to his specific parameters for what configurations of mass and chimney they can handle - among other things, they do always need a vertical exit chimney for both the secondary air and the bell-type heat storage to function.

... [further admiration for Peter's design]
Lastly, if one does fore go the mass and instead just has stove pipe exposed to open air inside the dwelling, would simply checking for creosote often solve the whole flue fire conundrum?

Checking the pipes regularly for creosote, troubleshooting and rectifying the burn or fuel problems if anything other than soot and ash is discovered, and routinely also checking for and re-sealing the system against smoke or CO leaks, possibly with a high-end CO detector as backup.

I'd seriously consider using something else purpose-built as a radiator for a small space not needing heat storage, rather than a squiggly chimney layout.
There are chimney heat-extractors, for example. Metal bells made of stronger steel, like cleaned-out fuel drums or oil cans, could be hooked up to offer more surface area in a compact space with fewer joints to inspect, among other advantages. You could easily attach heat-shielding or heat-storage jackets, internal or external heat-exchange fins, etc. If you're not going to get the benefit of a contact-friendly horizontal seating bench, and you don't have a heavy heat storage mass that needs to be spread out horizontally to reduce your foundation requirements, the advantages to running horizontal pipes are getting smaller.

I really thought that the complete burning of the wood eliminated the creosote issue.

It does, if you are consistent with your fuel quality and maintenance schedule. With an improvised design, you'd need to log and develop your own appropriate maintenance schedule. No stove can operate as designed if its orofices get clogged up or degraded so they're no longer the shape they were designed to be.

I guess part of the prejudice is also that historically-savvy fire and building inspectors know about the older heat-miser chimney-spaghetti experiments for ordinary woodstoves, and the catastrophic fires that result.
(They also know about the older barrel-stove designs for DIY woodstoves, and the catastrophic fires that result from these short-lived beasties, that are prone to spontaneous collapse while burning in Year 3 to 5 of operation.)

Building something that resembles a known fire-trap opens you up to greater prejudice. Even if you hit the nail on the head with respect to the biggest design problem that caused the older firetraps to fail, the cultural memory of danger causes people (including me) to remain prejudiced with fear and concern, and want extra reassurance that you're taking adequate, redundant, effective precautions against repeating the tragedy.

Because I've seen metal warp and degrade in the fire path, I look extra-close at anything metal located close to the fire. Peter's high-performing P-channels must submit to such inspection, along with all the newbie weld-o-matic experiments.
Because I've seen the historic articles about stovepipe-spaghetti chimney fires, and the current panic about CO poisoning in tightly-sealed homes, I am leery and look more closely at any exposed-pipe design suggestions.

It doesn't mean it can't be done. It means proceed with all due caution, and after looking into less-worrying alternatives.

Thanks for any feedback.

Sorry it took so long! Hope you're still interested. If you have settled on a project and built something, I'd love to see pictures.

Jacob Green : When I get all done with this, I expect that most of the frequent posters here at premies.com to nod their heads in a ''told you so manner! But this
is new material for many of us!

I often talk about a specific rocket mass heater RMH Build configuration and its effect on good Rocket builds. If you were able to build Two equal rocket mass heaters,
-except for the size of the Barrels placed over identical Heat Risers then both barrels have to give off the same amount of heat energy!

Counterintuitively, the smaller barrel -With a smaller Mass to Surface Area MUST give off the same amount of heat at a Higher temperature !

This is why when your 3 yr old is tired and wants up in your lap -they are such a good cuddle. With tear lower Volume to Skin Surface they Miust radiate the surplus at a
higher temp to maintain 98.6 than an Adult with a larger mass !

O. K., Now we can get serious about discussing the final diameter of the rocket mass heaters RMHs Burner base And the size off the final Thermal Mass Bench !

Irregardless of the material of the Final Total Mass of the RMH, There is an accumulating affect that occurs where the greater the 'Total Material Mass' of the Final build
The increase in the final diameter of the RMH's Burner Base, Or its Thermal mass The lower the final radiated temperature will be off of that Masses surface !

So, Automatically, any effort we make to add additional insulating layers between the RMHS Combustion Core and any Exposed potential Flammable surface -increases
the final Surface area, and reduces the temperature radiated off of that surface !

allen lumley wrote:Jacob Green : When I get all done with this, I expect that most of the frequent posters here at premies.com to nod their heads in a ''told you so manner! But this
is new material for many of us!

I often talk about a specific Rocket Mass Heater RMH Build configuration and its effect on good Rocket builds. If you were able to build Two equal Rocket Mass Heaters,
-except for the size of the Barrels placed over identical Heat Risers then both barrels have to give off the same amount of heat energy!

Counterintuitively, the smaller barrel -With a smaller Mass to Surface Area MUST give off the same amount of heat at a Higher temperature !

This is why when your 3 yr old is tired and wants up in your lap -they are such a good cuddle. With tear lower Volume to Skin Surface they Miust radiate the surplus at a
higher temp to maintain 98.6 than an Adult with a larger mass !

O. K., Now we can get serious about discussing the final diameter of the Rocket Mass Heaters RMHs Burner base And the size off the final Thermal Mass Bench !

Irregardless of the material of the Final Total Mass of the RMH, There is an accumulating affect that occurs where the greater the 'Total Material Mass' of the Final build
The increase in the final diameter of the RMH's Burner Base, Or its Thermal mass The lower the final radiated temperature will be off of that Masses surface !

So, Automatically, any effort we make to add additional insulating layers between the RMHS Combustion Core and any Exposed potential Flammable surface -increases
the final Surface area, and reduces the temperature radiated off of that surface !

This makes it harder to judge the final effect !

I think this answers your primary question !

For the good of the Craft ! Big AL !

All attempts to estimate the heat output of a given wood-fired heater are approximate. There are so many variables. If you look at commercial woodstoves, you'll see there's a range with a factor of two - e.g. 20,000 to 40,000 BTU - which is about as close as you can predict given the huge variability in chimney draw, wood type and condition, operator skill and diligence (and preferences), etc.

That being said, we each try to answer this question about how much heat a given system will give off with a combination of experience and guesswork/math formulae. Also known as "trial and error" - you try a bunch of things, you notice when the effect gets outside the boundaries of what you want i.e. error, and that's how you define the working parameters.

So speaking from experience rather than formulae:

- In my and Ernie's experience, a smaller barrel on a 6" J-type heater will tend to be a higher temperature than a big one, but it won't necessarily put out more heat altogether. The smaller barrel has the same amount of gas passing through it faster, and those constrictions create turbulence that may let the barrel contact more of the hottest gas and collect more heat. There's no physical law that says it has to get to a higher temperature, but the effect does seem to be that way.
A bigger barrel on the same firebox will let the hot gases hang out at the top longer, so they might be expected to deposit more heat. But they will also have more room to flow down the center of the space, leaving slow (and already-harvested) gas sluggishly drifting down along the sides in contact with the barrel. It might be the lack of turbulence, or the stagnation, as much as the larger surface area that makes these larger barrels radiate at a lower temperature. I would need to check the exhaust temps on both, accurately and repeatedly, to be sure whether one or the other puts more heat into the bench. I could guess, but I don't know.

Another thing we know from experience is that if you offset the barrel to one side or another, you get more heat delivery on the side with more gas flow. You might think it would be the small side - that gas would be squeezed against that side - but if you think about it there's no reason for the gas to "try" to get through the side with more resistance. It becomes a backwater, and most of the heat takes the main channel.

So: In practice it does seem to be true that the smaller barrel gives off energy at a slightly higher temperature, but not more total energy.
I would caution that there's no reason that the energy should be absolutely the same, however. Unlike your three-year-old, who has a sophisticated metabolism that regulates his core temperature (and a sophisticated, animated, socially-adept set of instincts and intelligence that allow him/her to extract food, water, cuddles, and whatever else it takes to maintain homeostasis and growth from his/her surroundings), the rocket stove is a very fancy pile of rocks.

The barrel size may also affect the system's total draw, and speed of burn. It could affect it in either of two ways.
More resistance in the narrow spaces or due to easier clogging up with ash could mean a slower burn. Or, if the smaller barrel puts the warm gas into the bench sooner, or is effectively closer to the chimney pipe (because it's smaller and higher temperature), it could results in a higher exhaust temperature, and that chimney draw could make it a faster burn.

The gap above the heat riser can make the difference between
- useful stratification (bell effect, delivering heat at the top and exhausting cooler gas at the bottom),
- random turbulence (more heat delivery to barrel top but also more drag and resistance to flow in the barrel itself)
- useful jet-like behavior (downdraft siphon, cooking hot-spot, or 'reburn' effects).

The whole system is dependent on the rest of the system.
We can come up with models and rules of thumb that seem to match our observations, but accurately modeling this many interdependent dynamic variables is kind of like trying to predict the weather closely or well in advance.

It is hardest to estimate the effect of the exit chimney and house this way, because each one behaves differently in different weather and times of day, let alone the easily-observed differences in shape, height, insulation, and degree of protection by the house or building. We often use rules of thumb from elsewhere in the industry, and then if they don't contradict our observations we go with it. We have seen plenty of systems that run fine in cooler weather, mornings and evenings e.g., that balk if started in the heat of the day. this is not surprising, but it makes considerations of the local climate and lifestyle essential if you are trying to get the whole system down to maximum efficiency, extracting as much heat as possible while still allowing successful draft.

Before you talk about estimating the size or mass of the back end, you have to be clear what your goals are.
- Are you interested in a heater which works reliably in all wind conditions, and all outdoor temperatures? Particularly if you want to be able to show the thing off during an 80-degree day, you will need to capture less heat and make sure your exit chimney is substantially hotter to be able to do this. In cold climates where people wear sweaters, and you don't light the stove if its 60F outside, you just bundle up, you can make the exhaust cooler and still get satisfactory draft.

- How much heat will your chimney/exit cause to be lost from the exhaust after it gets there? If you have an exposed outdoor chimney, it can get chilled down to dew point of the water in the exhaust, and cause a chimney stall.
I used to think that chimneys would only draft backwards if they "supercooled" - if they became colder than the outside air around them. I remember hearing theories about how the chimney was getting too cold due to wind chill, or being on the north side of the house, or something. But I've learned since then.
A couple of great points Peter made in conversation recently:
1) A kilo of firewood at 15% moisture (considered quite dry in most climates) will release 0.64 kilos of water in its exhaust. I ran the numbers and even theoretically-perfectly-dry wood at 0% moisture would put out something like 0.54 lbs. There's just that much hydrogen trapped in the fuel. Burning hydrogen and carbohydrates releases water. (H + O => H2O). (this is partly why light hydrocarbon fuels like propane and natural gas have a reputation for "wet" exhaust, they're relatively hydrogen-rich and can produce more than their own weight in water.)
2) Once the chimney is cool enough for water to condense, then a chimney stall can happen this way: Hot exhaust reaches a part of the chimney where it is cool enough for water to condense on the sides of the pipe. A dew forms and runs downward. When the dew reaches the lower part of the pipe or the heater mass, presuming it's hot enough, it evaporates again. Evaporation robs some heat from the pipes and exhaust. Now the rising exhaust is cooler, and has a higher water content. It rises up in the chimney, reaches that cool point again, and more of the water condenses. The process builds up water saturation, and moves the cold downwards in the pipe, until the exhaust is super-saturated with water and too cold and dense to rise in the chimney. At that point the chimney "stalls," and its burden of cold, dense, possibly-foggy exhaust falls out whatever low opening it can manage. This is usually the door or air controls of your stove/heater.
Because the hotter exhaust coming up behind may have been pushing up against this "cold plug" for some time, the effect can be like the smoke is "bouncing" up and down in the pipe. It will draw a little, then the whole mess falls back out in a plume of smoke. then it draws a little again... it can be like the pulses of an air-starved fire, but much slower and wetter.

The temperature at which the exhaust starts to condense varies with the dew point of water due to current relative humidity. The relative humidity of the exhaust will vary with how much extra air the fire is drawing (if any), and how wet that air and fuel are to begin with. In a humid climate, the air feeding the fire might be contributing a lot more moisture than in a dry climate, and the heat of the fire can temporarily reduce the relative humidity but doesn't eliminate the molecules of water themselves. It's all got to go somewhere.

I figure I've got it exactly right if the exhaust usually comes out of the stovepipe clear, maybe shows a white plume of "steam" (steam is clear, I'd call the white stuff fog) for a while in the cold air, then clears up again. I'm also OK with the exhaust just coming out clear. But if it's rolling out as a thick cloud of fog that falls downward from the chimney, I'm below that condensation point.

The masonry heater builders just try to stay up above the boiling temperature of water, roughly 200F / 100C.

There is a similar process with creosote if your exhaust is dirty, but it happens at more like 350 F / 200 C, and instead of evaporating again it might catch on fire.

Pick a desired chimney temperature.
Decide for yourself whether you're going to attempt a super-low exhaust temperature and deal with a possible chimney stall (bells and whistles, sideways-or-up exhaust options, etc),
or stay in the minimum range preferred by heater masons,
or just blast enough heat out that you don't need to worry about anything condensing, there's another consideration for the back end.

Pick a desired household temperature and how long you want it to stay there Do you want the mass to hold, and release, enough heat overnight to compensate for the entire heat loss of your building - that is, "heat the whole house?" A mass this big will take time to warm up, and may be capable of holding the whole house at a chillier temperature if not regularly fired. (This is nice in summer, not so great coming back from vacation in winter.) Big mass means very stable temperatures with slow changes.
Do you want it to merely hold enough heat to be warm in itself the next morning - that is, "stay warm overnight?"
Do you want it to just put out some random amount of warmth whenever you light it, and reduce the furnace bills or whatever - that is, "help me stay warm longer?"
Or do you want it to heat up quickly and responsively when you're there and feeding it, and also cool down relatively quickly after you leave - that is, "on-demand heat only?" Low mass means quicker temperature changes.

How much heat the mass can absorb from the exhaust is a question of its conductivity, surface area, and of arranging enough time or turbulence for the exhaust to deliver that heat.
How much heat the mass can release into the room takes conductivity (inside to outside surface, and to whatever it's touching) and emissivity (to release radiant heat).
How much heat it can store is a product of specific heat capacity and temperature - the amount of heat per pound the mass can hold per degree you warm it up, how many pounds you've got, and how many degrees you can raise its temperature.
How fast that heat gets from the core to the outside is mostly determined by how thin and conductive the material is. So a metal barrel radiates not quite instantly, where a 1" or 2" tile stove radiates within a few hours, and an 8" mass may reach its peak surface temperature at some point the following day.

Mass also affects foundation requirements, the size and shape of the project, and so on. There may be real limits to how much mass you can tolerate in a given situation, and reasons why you might want it to be one shape or another besides just the heat-delivery properties.

You can run the calculations using theoretical data from engineering.com, but you have to make some assumptions (like, what's the average temperature of your whole mass?) that aren't always easy to get reliable data about.
It's often easier to just say what's happened for other projects and build on that.

We estimate the heat storage lag time for cob (earthen masonry) at about 1" per hour, based on Ianto's rule of thumb and confirmed, roughly, by personal experience. It won't get cold that many hours later; it will be somewhere in the region of its peak heat output.

Both Peter and us Wisners tend to aim for a roughly similar exhaust temperature. We may be slightly lower. Peter wants the exhaust itself about 200 F / 100 C. We don't have as fancy equipment so we're making the pipe temps about 100F where the exit the bench (when the bench is new and wet, which is when we get to test most of our projects), or up to about 150 F once the mass hits operating temps. Touchable, warm not hot. This is probably very close to the same actual exhaust temps inside the pipes, given the range of operating conditions and so on. The exhaust from both tends to rise a few feet as it emerges into outside air, then streams out relatively neutral or slightly denser than air once it cools a bit.

A lot of Peter's test systems have been shop prototypes, and I don't know if he's got rules for how much mass a heater needs. Our prototypes don't always have mass, but the ones in buildings usually do unless it's a shop or occasional-use space.

We have developed a rule of thumb for estimating the useful length of the bench using smooth metal channels all in one line. Max length, minus 5' per 90-degree turn, plus a factor for good chimneys, minus a factor for cold starts/warm climates. For an 8" system we go with about 50' max (before subtracting elbows), more like 60' if the chimney is excellent. For a 6" system we go with about 40' max. This means in practice, with the usual turns and so on, they tend to come out 15-35 feet of actual pipe for an 8" system, and something like 12-30 feet for 6" systems.

Because of problems with systems where the cross-sectional area (CSA) varied with different-size and shape pipes in the same build, we now work with the same CSA throughout, except in that barrel/manifold area, or with controlled transitions to a very similar CSA in specific places.
We tend to wrap these in the minimum amount of masonry for structural integrity, which is 3" to 4", where there's plenty of clearance and you want a hot spot that gets hot sooner and reaches a higher temperature. We wrap them in more masonry (5 to 6") where we want even, overnight heating. Because the mass is usually kinda square, and the pipes are round, this means the thickness varies from about 4" to about 12", and gives a nice soft curve of overnight warmth.
This is a system optimized for small, residential use.

Peter has a system, which I've heard about other masons using, for estimating the heat exchange surface by the internal surface area (whether pipes or bells). There is a different surface area for different materials - for steel it would be about 8.8 square meters for his 8" batch box, and for brick or other clay masonry about 10 to 11 square meters.

I've heard some people suggest that bells are more efficient. Depends what you mean by efficiency. They trap the hot exhaust at the top, effectively sorting it so that you know that only the coolest exhaust is escaping up the chimney, and you can certainly get more surface area in a smaller footprint, and you can heat a mass up to a higher temperature more easily. But they are also taller, and building a dome or capping slab and the whole thing airtight and massive takes some skill. You can use a barrel as a bell, but that doesn't give you mass storage.
Also, the whole point of the low bench configuration is that you can sit on it. You may benefit more from a larger mass at a lower temperature, simply because you can sit on it and be comfortable and keep the house at a lower temperature overall. If you can drop the house temperature 10 degrees for the same comfort, as often happens with radiant floors and other low-aspect heat (because you don't stratify hot air at the top of the room, wasting heat to nobody's benefit) - that lower house temperature results in a smaller overall heat loss, and much less wood needed for the same comfort.

The bell also absolutely requires a vertical exit chimney (the benches usually perform better with one too, but have been built without). It's taller, and hollow (has to be at least 4x system size), so overall I think the space requirements are going to be roughly similar for a lot of configurations even if it does save floor footprint. Don't forget the bench can be built as a heated floor, so only the combustion area takes up actual real estate.

From what I've seen of Peter's larger 8" batch box thingy, and the bell-barrels used with it, it seems a great way to handle very large heating loads but unnecessary for efficient homes. I haven't played with the 6" batch boxes except for watching Matt's rough one this fall, and though it was smoking a lot I think it was partly peoples' handling of it rather than the box itself. I think the J-type stoves force people to a little higher standard of fire-tending, in a way, because you simply can't over-stuff it and go away. (fill it and go away, yes. Over-fill it, hard to do unless you go up outside the firebox.)

SO:
- For small, properly-insulated homes in the northern US, climates from 4000 to 8000 HDD, we are seeing good results with an 8" J-style firebox and about 3 to 5 tons of mass. this is a floor footprint like a sofa, bed, or a couple of loveseats.
- For tiny homes or warmer climates, one-room heaters for offices and guest cabins and such, the 6" J-style heaters are doing great with between 1/2 ton and 3 tons of mass.
Depending on the space, the heat may last just overnight or for up to 3 days.
If the space includes a well-insulated floor slab, masonry chimney, trombe walls, or other massive features, not all the mass needs to be in the heater itself. Some people use water tanks even.

I would guess that the batch boxes are roughly capable of heating like the next-size-up J-style, or maybe a bit more. The bench that our 8" firebox heated in Paul's shop, provided about 1/3 of the surface area Peter wanted for his 8" batch box. (the 8" didn't heat that bench very often, and the batch-box was never tested without the vertical chimney which the 8" didn't have.)

Update on this topic:
- We have seen evidence of creosote in 2 rocket mass heaters now.
One has definitely had at least one chimney fire, and possibly more than one.

Both of the stoves involved had some big, glaring problems as far as proportions, and did not meet the guidelines now published in our Builders Guide book.

In one case, the heater had relatively normal proportions except it had 14 elbows in the pipe between firebox and chimney. This is roughly double what we would normally recommend, and it causes the exhaust to slow down a lot. This stove is difficult to light, and touchy to operate.

In the other case, the heater was built using tricks from at least 3 different builders, in a workshop led by someone who subsequently stopped returning calls from the owner.
Combining tricks from different stove designs, without testing and follow-up, often results in a Frankenstein's monster with functional problems.
In this case, one trick (not recommended) involved forming the heat riser around wire mesh interior formwork. When this burned out, the inner lining of the heat riser crumbled away, leaving it much larger than it should have been. Combined with a too-short feed and some other dimensional errors, this led to the fire burning with a bad fuel-to-air ratio, and to over-powered draft that probably helped draw sparks all the way to the exit chimney to light off the resulting creosote.

- We recommend sticking with proven proportions, and proven combinations of features, wherever possible. for some reason, a LOT of builders want to re-design the rocket mass heater on paper without much (or any) practice..
- We recommend building a proper chimney, with proper clearances to combustibles, and proper structural installation (three to four screws per joint, through-ceiling and through-roof collars that support the weight of the pipes, etc). Modern manufactured chimneys, when installed correctly, will protect the surrounding structure in the event of a chimney fire.
- We recommend inspecting any stove or fireplace or working chimney at least once per year, regardless of the length of your heating season. (Folks in warm climates don't get to stretch it out, because a seldom-used chimney is even more likely to have problems than a routinely-used one. Folks in very cold climates are welcome to inspect more often if they see fit.) Remember that DIY chimney maintenance came into fashion within living memory for most of the folks who raised us; get a professional chimney sweep if you can.
- If you find creosote (sticky or crumbly black material) in any masonry heater or rocket mass heater, or its chimney, there is a problem.
It could be poor design, air leaks/poor construction, poor fuel, or operator error (such as shutting down the fire before the fuel is finished burning). The problem should be diagnosed and fixed, and the creosote removed, before continuing to use the heater.